1. Field of the Invention
This invention relates to a four stroke engine and more particularly to an air induction system of a four stroke engine, which is most suitable to an outboard motor.
2. Description of Related Art
Recently, some outboard motors incline to utilize four stroke engines. One reason for this tendency is that emissions from the four stroke engines are clean rather than those of two stroke crankcase compression engines. Also, usually the engines have multiple cylinders in order to produce relatively large power. The respective cylinders are spaced vertically relative to each other in those engines for outboard motors. That is, cylinder bores extend generally horizontally.
An air induction system is provided for introducing air charge to combustion chambers in the cylinders. The air induction system has air intake ducts extending generally horizontally along the cylinder bores and a common plenum chamber placed upstream of the air intake ducts. It is desirable to make the air intake ducts proper lengths for improving engine power, particularly the torque characteristic under acceleration conditions from low or medium speeds by using the inertia charge effect. Also, the plenum chamber has a certain volume and a height so that the air intake ducts are connected thereto.
On the other hand, however, the engine is encircled with a protective cowling and a number of engine components must be placed in narrow room formed between the engine body and the protective cowling. Under the circumstances, it is a problem how to make sufficient space for placing the engine components as well as the air intake ducts and the plenum chamber.
It is, therefore, a principal object of this invention to provide a four stroke engine wherein a certain space can be available for placing engine components other than the air intake ducts and the plenum chamber.
Also, as described above, the air intake ducts must have certain lengths. In the meantime, usually a throttle valve for admitting air charge to combustion chambers is contained in a throttle body placed upstream of the plenum chamber. Due to this arrangement, lengths between the throttle valve and the respective combustion chambers tend to be relatively long. Thus, the engine cannot response so quickly to the operator's desire. Accordingly, the operator is likely to have bad feeling in engine operation.
It is, therefore, another object of this invention to provide a four stroke engine that can response quickly to the operator's desire in engine operation.